A complex image forming apparatus which is provided with a copy mode function for optically reading an image of a document set on a document stand and printing the above read image on paper and a printer mode function for printing an image transferred from an external device, for example, a personal computer, on paper, is known.
In the complex image forming apparatus, there may occur some jobs simultaneously such as a copy JOB by use of the copy mode function and a printer JOB by use of the printer mode function. In this case, these jobs are executed sequentially.
When there is no paper of the proper size for a first JOB, the first JOB cannot be started. When a user feeds the proper sized paper, the first JOB is started, and when the first JOB is completed, the second JOB is started.
In this case, even if there is no proper sized paper for the first JOB, when there is proper sized paper for the second JOB, the second JOB is thought to be executable. The second JOB, however, cannot actually be started until the first JOB is completed. This waiting time is of no use, and may delay or frustrate the user.